There exists a number of commercially available lasers capable of generating short pulses of radiation having a duration in the picosecond range. In order to develop, test, optimize and utilize such lasers, it is necessary to measure the length of these short pulses. Pulses in the picosecond regime are too short to be measured by conventional photodetectors. Accordingly, devices have been developed for measuring these pulses which rely on a technique known as autocorrelation.
Autocorrelation can briefly be described as a measurement system for developing a composite image of a periodic waveform through combination of the measured signal with itself over a range of time delays between the signals. The beam of light consisting of a stream of pulses is divided into two parts and recombined later downstream. Prior to recombination, a varying time delay is introduced between the two beams. When the beams are rejoined the magnitude of the combined beam provides information about the original signal.
When the two signals are identically synchronized (i.e. with peaks overlapping), the combination will produce a pulse train of essentially equal pulses. If, however, the two pulse trains are out of sync by more than their pulse width, the signal will be zero. When the pulse trains are out of sync by a fraction of their pulse width, the product will be greater than zero but less than when the pulse trains exactly coincide.
The object of autocorrelation is to produce a series of varied delays between two pulsed beams. At each selected delay, the combined signal is measured. A series of such measurements provides an image of the pulses which can be displayed on an output device such as an oscilloscope or computer printer.
In prior art devices relying on the autocorrelation technique, the two pulsed beams are typically recombined in a second harmonic generator crystal. When the pulses in two beams overlap, the crystal will generate a second harmonic output. The amplitude of the second harmonic output is detected with a photodetector. While the photodetector can not respond fast enough to record the combination of any particular pair of pulses from the two beams, it will generate a signal proportional to the combination of the pulses averaged over a series of pulses. The level of this averaged output signal will vary as the time delay is varied. The output signal of the photodetector is supplied to a oscilloscope.
In the prior art devices relying on an autocorrelation technique for measuring pulse width, some means must be provided to delay one of the two optical beams. One approach for introducing an optical delay is to direct the beam through a rotating glass block. As the block rotates, the path length of the beam is varied such that a varying time delay will be produced.
One example of a device using a rotating glass block can be found in U.S. Pat. No. 4,406,542 issued Sept. 27, 1983, to Boggy et al. In this particular device, both of the beams are directed through the rotating block to insure that a substantially linear time delay is introduced. As stated in the Boggy patent, it is extremely desirable to produce a linearly varying delay in order to facilitate proper calibration on the oscilloscope. As can be appreciated, an oscilloscope has a uniform rate of sweep in the horizontal direction. If the variation of the delay between the two beams was non-linear, the waveform on the oscilloscope would be distorted and have to be corrected. Accordingly, by generating a linearly varying delay, calibration and measurement are substantially simplified.
The autocorrelator described in the Boggy patent is designed to measure pulse widths between 1 and 100 picoseconds in duration. In order to measure the length of longer pulses, the block would have to be made thicker to create greater delays. Unfortunately, if the block was made thicker, it would also become heavier which could create problems in mounting the block and maintaining a constant speed of rotation. The Boggy device is also incapable of measuring shorter pulses because of group velocity dispersion effects which occur in the glass block. For these reasons, the device described in Boggy is limited to measurement of pulse widths over a relatively short range. It would be desirable to provide a device which can measure both relatively short pulse widths as well as significantly longer pulse widths. To applicant's knowledge there is no device on the market which can provide autocorrelation for pulse widths over a range between 50 femptoseconds and 350 picoseconds.
Other prior art autocorrelators are described in U.S. Pat. No. 4,472,053 issued Sept. 18, 1984 to Wyatt et al. and U.S. Pat. No. 4,628,473 issued Dec. 9, 1986 to Weaver. In both of the latter two patents, a delay is introduced into the signal without moving parts. In the Wyatt design, a diffraction grating is used to introduce a continuous differential time delay along the spatial width coordinate of the beam. In the Weaver device, the beam is passed through an electrooptic crystal whose index of refraction is responsive to an electric field. As the index of refraction is varied, the delay of the beam passing through the crystal is varied. Both of the latter devices are also limited to measuring the duration of pulses over a relatively narrow measurement range.
Accordingly, it is an object of the subject invention to provide a new and improved autocorrelator apparatus.
It is another object of the subject invention to provide an improved means for delaying an optical beam in an autocorrelator device.
It is a further object of the subject invention to provide a new and improved autocorrelator that can measure pulse widths from 50 femptoseconds to 350 picoseconds.
It is still another object of the subject invention to provide a new and improved autocorrelator which includes a means for translating a stage at a constant velocity.
It is still a further object of the subject invention to provide a means for varying the path length of an optical beam.
It is still a further object of the subject invention to provide a means for changing constant rotational movement into constant linear motion.
It is still another object of the subject invention to provide a novel cam and cam follower arrangement that imparts constant linear motion to a stage carrying a reflector in order to create a delay in an optical beam which varies constantly over time.